s/regex/string/

##</code><code>##

my $line = 'abbc';
$line =~ s/(.)\1/XX/g;
say $line;

--output:--
aXXc
##</code><code>##

my $line = 'abbc';
$line =~ s/(.)/X$1X/g;
say $line;

--output:--
XaXXbXXbXXcX

##</code><code>##

my $line = 'abbc';
$line =~ s/(.)/$1$1/g;
say $line;

--output:--
aabbbbcc
##</code><code>##

$str = aa;
$str =~ /(a)$1/;
print "$&";